Computers have been designed with accessible bus connectors that provide external electrical connections to their buses. This configuration is most common in portable and specifically notebook-size computers and is used in two scenarios. The bus connector may be used to connect the portable computer to a non-portable docking station that many times allows connection to a larger display and a full size keyboard. In other implementations, the bus connector is adapted to be connected to somewhat portable dock units that can contain hard drives, CD ROMS, and power supplies. Compatible dock units are available that have larger battery power supplies, CD ROMS, and other capabilities for multimedia functions, for example.
One issue that arises when providing this docking capability is how to handle the physical electrical mating between the computer, or primary unit, and the dock unit. Glitches or anomalous voltages will arise on the primary bus when the primary bus makes electrical contact with the corresponding connector on the dock unit's bus extension. Such glitches can cause the primary unit to hang or become inoperable, requiring rebooting.
One solution to handling the docking and the resulting glitches on the bus is to render docking illegal when the primary unit is operational. This solution solves the problem of compensating for the glitches, but is somewhat unacceptable to most users. Another solution is to provide buffers between the primary bus and the primary unit's bus extension. During operation, these buffers act as relay agents that simply pass-on valid bus activity between the primary bus and extension in the dock unit. During docking, however, they prevent the glitches from appearing on the primary unit's bus and, thus, insulate the primary unit. The problem with the buffer solution is two-fold. The buffers: 1) add expense to the primary unit since they represent an additional hardware system; and 2) slow transactions between the primary bus and the dock unit's bus extension.
Another approach is to generate a dock request signal just prior to the physical connection between the primary unit's bus conductors and those of the dock unit; the primary unit electrically floats the primary bus in response to receiving this request signal. The solution, while allowing hot docking or docking while the primary unit is operational, avoids the delay and costs associated with the hardware buffers. The problem, however, is that established protocols do not allow the docking of a second dock unit to dock/primary unit pairs.